Citrus greening, also called Huanglongbing or yellow dragon disease, is a disease of citrus. This bacterial disease is thought to have originated in China in the early 1900's. The disease is primarily spread by two species of psyllid insects. One species, the Asian citrus psyllid, Diaphorina citri, has been present in Florida since 1998. The bacteria itself is not harmful to humans but the disease is damaging to the citrus crops. There are three strains of the bacteria: an Asian version, an African version, and a recently described American strain discovered in Brazil.
The Asian strain, Candidatus Liberibacter asiaticus (Ca. L., Las), was found in Florida in early September, 2005. As a result, citrus greening disease is becoming a major threat to the U.S. citrus industry. Other than tree removal, there has been no known effective control once a tree is infected and there has been no known cure for the disease. Infected trees may produce misshapen, unmarketable, bitter fruit, or no fruit. Citrus greening reduces the quantity and quality of citrus fruits, eventually rendering infected trees useless. In areas of the world affected by citrus greening the average productive lifespan of citrus trees has dropped from 50 or more years to 15 or less. The trees in the orchards usually die 3-5 years after becoming infected and require removal and replanting. An infected tree produces fruit that is unsuitable for sale as fresh fruit or for juice.
Citrus plants infected by the citrus greening bacteria may not show symptoms for years following infection. Initial symptoms frequently include the appearance of yellow shoots on a tree. As the bacteria grow and move within the tree, the entire canopy progressively develops a yellow color.
The most characteristic symptoms of citrus greening are a blotchy leaf mottle and vein yellowing that develop on leaves attached to shoots, providing the overall yellow appearance. These foliar symptoms may superficially resemble a zinc deficiency although the green and yellow contrast is not as vivid with greening as it is with zinc deficiency or another disease, citrus variegated chlorosis. Leaves with citrus greening have a mottled appearance that differs from nutrition-related mottling in that greening-induced mottling usually crosses leaf veins. Nutrition related mottles usually are found between or along leaf veins and leaves may be small and upright.
Fruit from diseased trees are small, often misshapen, and typically some green color remains on ripened fruit. On Mandarin orange, fruit may develop an uneven ripening such that they appear half orange and half yellow. This symptom is the origin of the common name “greening.” Yields are almost minimal, and any developed fruit is rendered worthless due to small size, poor color, and bad taste.